It is well known in the art of chemical processing that catalysts can be prepared in a wide variety of shapes and sizes which will vary depending upon the particular reaction in which they are employed. For example, it is well known that the process of heterogeneous catalysts requires the presence of the catalyst in the form of discrete particles through which the reacting products may be passed under conditions necessary to effect the desired conversion. Depending upon the nature of the process, the discrete particles may be positioned in a fixed bed, moving bed or suspended in the reactants themselves as in a fluid bed process.
In some instances catalysts, such as compounds in the form of metals or other compositions, are required to be supported in their catalytic state since discrete particles in bulk form are neither sufficiently active or are so finely divided as not to be suitable for the intended use. A wide variety of catalytic supports are disclosed in the literature as well as processes for fabricating such supports and their use in chemical reactions.
Catalyst supports and the catalysts themselves can be fabricated in a wide variety of shapes and sizes with the optimum configuration depending upon the particular reaction in which the catalyst is employed. Batch operations may dictate the use of one type of supported catalyst whereas if the same reaction is conducted in continuous form a different configuration of the same catalyst may be preferred. Likewise, whether the reaction is effected in the liquid or gaseous state will of course, influence the choice of catalyst and particularly any support on which the catalyst may be contained.
Thus, one may find catalysts, both supported and unsupported, in the shape of discrete solid particles, porous particles, granules, spheres, microspheres, pellets, columns, rods, solid cylinders, hollow cylinders, porous sheets, screens meshes, honeycomb structures, and additional configurations or combinations thereof.
In many instances the mechanical strength of the catalyst or the support on which it is contained is not important and reaction conditions and reactants themselves are sufficiently mild so that the catalyst can be employed in a variety of shapes. For example, for some applications the catalyst can be prepared in situ or just prior to use and the structural strength or its particular configuration is not important.
However, for many reactions a serious problem can arise with shaped catalysts and catalyst supports if their structural strength is limited. For example, moving and shipping of catalysts which are prepared at a location other than the site at which they are used, in many instances requires that the catalyst or the support on which it is contained, be of sufficient structural integrity to withstand such motion. Additionally, charging and installing the catalysts to the catalyst zone of processing equipment and the motion of the equipment during operation can cause fragmentation of the catalyst and any supports with the production of unecessary particles or fines.
In moving bed processes the fines created due to fragmentation may be removed either in the existing fluid and unadvoidably discharged into the air or sewage system. Conversely, in fixed bed processes which utilize such catalysts, the fines resulting from poor structural strength of the catalyst or support may lead to plugging of the reactor which requires shut down of the process and removal of the fines.
A variety of methods have been proposed in the literature for improving the structural and mechanical strength of catalysts and the supports on which they are contained. For example, in U.S. Pat. No. 4,142,994 which issued on Mar. 6, 1979 and is assigned to Filtros Corporation, there is disclosed and claimed mechanically and thermally stable porous calcined shaped catalyst supports in the form of particles which do not easily fragment as compared to structures previously known and composed of similar materials. In the disclosed process a clay is shaped, acid leached to a degree insufficient to destroy the plasticity of the clay, formed into shaped particles, and the calcined particles further extracted without impairing their shape. The resulting catalyst supports have high pore volume and surface area, and sufficient mechanical and thermal strength to minimize fragmentation.
Other methods have been reported in the literature for the fabrication of catalyst supports having improved properties. For example, in U.S. Pat. No. 4,042,738 which issued Aug. 16, 1977 and is assigned to Corning Glass Works, a support in the form of a honeycomb structure having a plurality of interconnected partitions is provided and which is indicated to possess high thermal shock resistance. It is also indicated in this patent that the particular geometry of the honeycomb disclosed was selected to avoid a rigid structure, since a mechanically stiff catalyst support which is not readily deformable does indeed have a high structural modulus, but consequently also has a low thermal shock resistance. Hence for the particular process in which the honeycomb structure is used, i.e., in a catalytic converter for emission control of internal combustion engines, a support is needed which has a low structural modulus and high thermal shock resistance.
It should also be noted that while improvements in the structural strength of catalytic supports are not difficult to achieve and can be accomplished by a variety of means, it is usually only done at the expense of the overall catalytic activity. For example, lowering the porosity by compressing or compactation will in some instances result in a support having a greater mechanical strength. However, the decreased porosity results in a corresponding decrease in surface area and hence the amount of available sites for catalysis to occur.
In U.S. Pat. No. 4,366,093 which issued Dec. 28, 1982 to K. Shiozaki et al of Japan, a cylindrical molded catalyst is disclosed for use in fixed bed reactors. The molded catalyst is indicated to have a low resistance to fluids, a large effective surface area good heat conductivity and sufficient mechanical strength for most applications. The novelty of the disclosed invention appears to reside in the geometric shape, size and configuration of the cylindrical molded catalyst. It is further indicated therein that it is necessary that the cylindrical molded catalyst have a specific size, that is, it must be from 3 to 6 mm in outer diameter, at least 1.0 mm in inner diameter, a wall thickness of at most 1.5 mm and a height of 3 to 6 mm. It is further indicated at column 2, lines 3-10 of the patent that no catalyst of this particular configuration had ever been prepared and put to practical use since it had been considered that a cylindrical catalyst of the shape indicated above, would have insufficient mechanical strength. The patentees further disclose at column 2, lines 21-23 that the compressive breaking strength in the direction of the diameter of the circle is at least 0.2 kg. It is further indicated that the materials used in the preparation of the cylindrical molded catalyst can be alumina, silica, or mixtures thereof and the catalytically effective material can be a metal salt or metal oxide such as copper halides, copper oxides and the like.
Hence prior to the present invention the catalysts, both supported and unsupported, disclosed in the literature were of a variety of shapes and sizes and in many instances custom made to serve the particular reaction conditions employed in the process being used. A catalyst support system of a particular configuration and shape which was useful for one type of process was not necessarily useful for others. Moreover, cylindrical molded catalysts as in the above patent did not always have sufficient crush strength to overcome the disadvantages noted and provide the optimium degree of catalytic activity. Efforts to improve the crush strength were directed largely to fabricating the cylindrical supports with thicker walls, or adding materials to the support which increased its mechanical strength. Catalyst supports which had increased wall thickness, resulted in a decrease in the inner core diameter and hence, a reduction in catalyst surface area. Catalyst supports which contained strengthening materials sacrificed a reduction in porosity and likewise a reduction in active surface area. What would be advantageous, would be a catalyst support which has sufficient structural integrity and yet which has no reduction in catalytic activity.